Different modes of operation of terminal devices are known from the radio telephone network according to the TETRA (TErrestrial Trunked Radio) standard. Modes of operation are e.g. the so-called direct mode operation, that is, the terminal device's DMO (Direct Mode Operation) function, which means direct communication taking place between terminal devices independently of the TETRA network proper. The DMO function is utilised especially e.g. in peripheral areas of the coverage area of the TETRA network or outside this, where the network's field strength is weak or the network is not at all available. The DMO function may also be used within the coverage area of the network, if the group of users wishes e.g. to communicate only internally and does not wish that its communication is audible to all others communicating in the network.
Furthermore, in the TETRA network a so-called mode of operation supporting distribution of network resources, that is, the terminal device's TMO (Trunked Mode Operation) function, is available in the coverage area of the physical network formed by base transceiver stations (generally at least in densely populated areas). Use of this function is possible for all users of terminal devices located within the coverage area of the TETRA network. Besides transmission of voice messages, it is possible in the TMO function e.g. to form groups of users, whereby messages may be allocated to the relevant group. In addition, in the physical TETRA network transmission of so-called status and SDS (Short Data Service) messages is possible as well as transmission of group settings, for example, by units forming a group or by a dispatcher, who usually is an operation leader on duty at a piece of fixed terminal device. Such messages include e.g. short messages and alarms, and settings include forming and management of groups performed by an operator over the air.
However, problems in smooth communications are caused by e.g. the edges of the TETRA network's audibility coverage area and by the areas remaining outside the coverage area. Maintaining communication ability between units would be priceless, for example, in remote areas, where the actual physical TETRA network is not available due to its limited coverage area and where a group using the DMO function would nevertheless have a significant need to communicate, for example, with an operation leader working at a fixed terminal or with a mobile operation leader.
Examples of typical situations where functionality of the described kind in message transmission would often be needed are various kinds of rescue duties to be performed outside the TETRA network's coverage area, and in densely populated areas e.g. operations to be performed in basements and in parking halls of buildings made of concrete, where problems may occur with the covering ability of the actual physical TETRA network. Even within the coverage area of the actual physical TETRA network such situations may occur, when e.g. the operation leader would need to communicate from his terminal device with a group, which has chosen the DMO function and which is “isolated” in terms of communication. Furthermore, message transmission between groups operating in the same mode of operation within the TETRA network's coverage area is difficult, which is due, for example, to the different access permissions granted to the units, because it must in most cases be handled at the operational centre through the person on duty.
Some such gateway solutions of the TETRA network are known previously, where e.g. a special gateway device is arranged in a vehicle. Using this device it is possible e.g. from outside the coverage area of the actual physical TETRA network to transmit communication of all terminal devices using the DMO function to terminal devices located within the coverage area of the actual TETRA network. However, the implementation of the device is quite complicated, disadvantageous and specific. In addition, a device of the described kind transmits not only communication in speech form, but in practice also all the data to be transferred in the TETRA network, that is, e.g. the status messages of groups etc. For example, in the situations described above it would be sufficient to transmit communication only in speech form between units, for example, from groups using the DMO function of their terminal devices to a group located in the coverage area of the actual TETRA network and using the terminal device's TMO function, and vice versa.
WO publication 96/11554 presents a TMO-DMO relaying station using two receiver units and one transmitter unit as well as a special management unit to control these. The first receiver unit receives radio messages from a base transceiver station and the second receiver unit receives radio messages from terminal devices transmitting as a DMO function. However, such use of two receiver units makes the solution complex and disadvantageous.
Other solutions known from the analog technology side are various solutions of the gateway type. WO publication 94/28641 presents a relaying station suitable e.g. for an analog mobile telephone system (e.g. NMT). Here the connection from the receiving side to the transmitting side is in the audio frequency mode. The solution is purely of the repeater type, which cannot thus be used for relaying communication e.g. between terminal devices operating in different modes of operation.
In view of the known technology there are no advantageous and simple ways in a radio network system according to the TETRA standard e.g. of implementing communication in the described kind between the TETRA direct mode operation DMO and the TMO mode of operation supporting distribution of network resources.